1. Field of the Invention
This invention relates to the recovery of polyester base from scrap materials such as photographic films and other articles which normally have polymeric coatings applied to them during manufacture. More particularly, it relates to the recovery of the polyester material by subjecting the scrap materials to controlled conditions of high shear to remove the coatings.
2. Description of the Prior Art
Polyethylene terephthalate (PET), or polyester as it is commonly known, has been widely used as a base material for a variety of applications such as in photographic films, soda bottles and recording tapes. In many applications, the polyester is coated with binders, adhesives and metal compounds.
In the manufacture of photographic films, for example, the PET film base is coated with an adhesive polymer layer of a polyvinylidene chloride (PVCD) based resin commonly referred to as saran. This polymer layer is needed to permit the adhesion of the light-sensitive emulsion layer, usually a silver halide dispersed in gelatin, to the PET base since the gelatin does not bond well to a clean polyester surface. A saran layer may also be used in the manufacture of PET soda bottles and other PET articles.
Millions of pounds of polyester are scrapped each year because the presence of the PVCD coating inhibits recovery. It is obviously quite desirable to convert this scrapped PET into clean polyester which can be recycled as raw material in the production of polyester feedstocks for fibers, films and other articles. In the case of photographic film scrap, it would also be desirable to recover the silver in the halide emulsion. Recovery of the polyester and, in the case of scrap photographic film, of the silver in forms which are reusable reduces solid waste disposal problems and conserves our natural resources through recycling of these materials.
Prior art methods for recovering PET base material from photographic film, for example, have included treating the scrap material with hot caustic alkali solutions and with solvents. U.S. Pat. No. 3,652,466, which issued Mar. 28, 1972, to E. R. Hittel and assigned to the same assignee as the present application, is directed to a polyester recovery process which involves treating small pieces of coated film with a caustic aqueous alkali solution to form a slurry. The slurry is then fed down into a classification column wherein the countercurrent flow of a slowly rising aqueous liquid separates the PVCD coating from the polyester pieces.
In U.S. Pat. No. 3,873,314, which issued Mar. 25, 1984, to J. T. K. Woo et al, scrap photographic film is treated by solvents such as dimethylformamide (DMF) and tetrahydrofuran (THF). The mixtures as subjected to low shearing action by stirring to dissolve the halide emulsion, and PVCD resin layer. In both U.S. Pat. Nos. 3,503,904, which issued to H. J. Dietz et al on Mar. 31, 1970, and 3,928,253, which issued to J. S. Thornton et al on Dec. 23, 1975, the film scrap is treated with an alkaline glycol such as ethylene glycol to remove the PVCD subcoating and the photographic emulsion layers. As in the case of the foregoing prior art patents, the mixtures are agitated through low shearing action.
The prior art processes suffer from several disadvantages which make them economically infeasible. In the case of photographic films, a significant amount of silver is not recovered.
Moreover, merely subjecting small pieces of scrap film to a hot caustic or a solvent, and stirring the mixture in the manner described by the prior art causes the pieces to stick together when they contact one another so that a high degree of agglomeration occurs. This makes it extremely difficult to clean the individual polyester pieces sufficiently to permit recovery of commercially pure polyester.
The present invention overcomes the limitations and disadvantages of the prior art by providing a recovery process by which essentially pure polyester and indeed other plastic base materials may be recovered efficiently and economically through the effective use of high shear conditions. The silver values in scrap photographic film can likewise be recovered by subjecting the scrap film to similar high shear conditions as described and claimed in copending application Ser. No. 693,252, entitled "Recovery of Silver From Photographic Films by High Shear Conditions", filed the same date and by the same applicants as the present application.